This invention relates to color display systems which employ one or more electro-optic display devices such as liquid crystal display devices, and more particularly relates to DAC driver circuitry employed in such a system to convert incoming digital display signals to analog signals, and to address the display device with such analog signals.
Color display systems are known in which light bars of different colors are sequentially scrolled across a single electro-optic light modulator panel to produce a color display. See, for example, commonly assigned U.S. Pat. No. 5,532,763, incorporated herein by reference.
These display systems are particularly suitable for displaying color information in the form of continuously updated image information signals arranged in successive frames, such as color video information, in which each frame is composed of component color sub-frames, eg., red, green and blue sub-frames.
These systems employ an electro-optic light modulator comprised of a row-and-column matrix array of pixels, for modulating light in accordance with the image information signals during successive frame periods. The signal information is applied to the pixel rows of the array a line at a time during each frame period. Such a matrix array is preferably "active", ie., each pixel is connected to the array via an active switching element, in order to prevent cross-talk between pixels.
Because each sub-frame of information in a three color system must be addressed on a single electro-optic light modulator panel during one frame period, the addressing rate is three times faster than the rate for a three-panel system. At present, a reflective active-matrix liquid crystal display (AMLCD) on silicon employing a liquid crystal exhibiting the twisted nematic (TN) effect and transistors as the active switching elements, is the preferred electro-optic light modulator. Such a reflective device can support a high density of pixels, due to the fact that the wiring and switches can be integrated on the silicon substrate beneath the reflective electrodes. Moreover, such a device can be addressed at a much higher rate than prior transmissive devices.
Nevertheless, the fastest response time of the TN LC material, the time needed to drive the LC material to the dark state, is on the order of 100 microseconds, while the time for addressing a row is only 5 microseconds. Moreover, the pixel capacitance varies with both the applied voltage and time.
As a result, the accuracy of the display data transfer depends on the residual state of the previously stored information. This means that the brightness of a particular color depends on the brightness of the previous color. Two-dimensional look-up tables are presently used to provide correction values to the new signals, in order to avoid such color artifacts.
In published PCT application WO 96/00479, the problems of slow response time and varying pixel capacitance with voltage of a TN LC material in a single panel color sequential projector is approached by substituting an electro-optic material with a faster response time, and with capacitance subsantially independent of voltage, such as a ferroelectric LC material.
However, ferro-electric LC materials and similar electro-optic materials have a memory effect. Thus, WO 96/00479 teaches the use of a "blanking pulse", ie, an auxiliary signal, to reset the pixels prior to each new addressing with display data. Such a blanking pulse is applied during some part of the line selection period via either the row electrode or the column electrodes in combination with a reference electrode in the case where two-terminal devices such as diodes are used as the active matrix switching elements, or via a common counterelectrode in the case where three terminal devices such as TFTs are used as the active matrix switching elements.
Such a scheme requires separate means for generating the reset signal, either directly or as a combination of an auxiliary voltage and a reference voltage via a separate set of reference electrodes. Such circuit means are not readily integratable into drive circuitry which is designed to convert incoming digital information signals (eg., digital tv signals or computer display signals) to analog signals suitable for driving the display.